Hey Lap,
They are found in the wild in groups of one or two males to several females. Unfortunately, they usually try to murder each other in captivity. It's no accident that wholesalers always keep them in deli cups or each to their own cube.
I kept a pair of royal grammas before in a large tank like yours for many years--they hung out together in a cave and were very large, nearly 3". I wish I could tell you how they were paired, but I started taking care of the tank several years after it was set up. Unfortunately Royal Grammas are NOT like clownfish--you cannot just put any two juveniles together and be assured of a male-female pair because their sex is determined at conception and they are not hermaphrodites. Likewise, there is no clear way to tell the difference between a male and a female. Even if you do get a male and a female, there is no assurance they wouldn't just kill each other anyway.
You might try adding 2 juveniles to your tank and crossing your fingers. I've heard that adding a large one and a small one can help, but you are still rolling the dice as far as selecting a male and a female that will get along.